In general, a liquid crystal cell containing liquid crystals and a polarizing film are basically needed to manufacture a liquid crystal display device. To attach these together, a proper adhesive layer is required. To improve the functions of a liquid crystal display device, a retardation plate, a wide-viewing angle plate, or a brightness-enhancing film can be additionally laminated on a polarizing plate.
A liquid crystal display device is generally composed of an evenly arranged liquid crystal layer; a multiple structure polarizing plate, wherein a liquid crystal cell comprising a glass plate containing a transparent electrode layer or a plastic board is coated with an adhesive layer; a retardation plate; and a supplementary functional film.
The polarizing plate contains an iodine compound or dichroic polarizing material arranged in a certain direction, and a multi-layered structure is preferably formed using a triacetyl cellulose (TAC) protective film to protect the polarizing element. The polarizing plate might additionally include a retardation film having a unidirectional molecular arrangement, or a wide-viewing angle film such as a liquid crystal film.
The above mentioned films are composed of materials with different molecular structures and compositions, so that their physical properties are different. Especially, under a specific thermal and/or humidity condition, materials having an anisotropic molecular alignment shrink or expand, cause the lack of dimensional stability. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused the thermal and/or humidity condition remains, so that light leakage occurs at the region where the stress is concentrated.
To reduce light leakage, it is necessary to reduce the contraction of a polarizing plate under hot or humid conditions. However, it is still very difficult to eliminate stress from a liquid crystal panel loaded with a polarizing plate composed of materials with different physical properties.
The generally acceptable adhesives are rubber-based, acryl-based and silicon-based adhesives. Among these adhesives, the acryl-based adhesive has been the most widely used as a high-functional adhesive composition owing to its properties. To enhance durability, the molecular structure has to be properly modified, particularly in adhesive constitution, molecular weight and molecular weight distribution, cross-linking density, etc. In that case, an adhesive has to have high cohesive strength.
An adhesive layer is loaded on a polarizing plate for attachment to an LCD. If this LCD is used for a long time under hot or humid conditions, contraction of the polarizing plate will be observed and thus stress will be centered thereon. To overcome this problem, an adhesive is needed that allows for stress relaxation.
Japanese Laid-Open Patent Publication No. Hei 1-66283 describes a method of preparing an adhesive polarizing plate, in which an acrylic polymer composed of a (meth)acrylic acid alkyl ester with a C1˜C12 alkyl is loaded on the surface of a polarizing plate, and the adhesive layer contains an acrylic polymer composed of 15 weight % of a polymer having a weight average molecular weight of less than 100,000 and 10 weight % of another polymer having a weight average molecular weight of at least 1,000,000. According to this description, the durability of the adhesive is satisfied owing to the high content of the polymer, but a stress relaxation effect has not been shown, suggesting that stress is still centered by the change of polarizing plate size resulting from the high cohesive strength (coefficient of elasticity) of the adhesive.
Korean Patent Publication No. 1998-079266 describes how light leakage is reduced by loading an adhesive composition having a stress relaxation function on a polarizing plate, in which the composition is composed of 100 weight part of a high molecular weight acrylic copolymer having a weight average molecular weight of at least 1,000,000, 20˜200 weight part of a low molecular weight acrylic copolymer having a weight average molecular weight of up to 30,000, and 0.005˜5 weight part of a multi-functional cross-linking agent. However, according to this description, the content of the low molecular weight acrylic copolymer having a molecular weight of up to 30,000 is too high, whereas the compatibility is too low, so the low molecular weight molecules are inclined to move to the surface as time goes by, suggesting that durability is reduced under high temperature and high humidity conditions, with the generation of air bubbles or exfoliation. However, there is no description of a technical method to regulate a composition composed of low molecular molecules to improve durability.
Japanese Laid-Open Patent publication No. 2000-109771 describes a method to reduce light leakage by using an adhesive composition for a polarizing plate composed of a high molecular weight acrylic copolymer harboring a functional group which has a weight average molecular weight of 600,000˜2,000,000, 0.1˜50 weight part of an acrylic copolymer harboring a functional group which has a weight average molecular weight of up to 500,000, and up to 5 weight part of a multifunctional cross-linking compound in order to have a stress relaxation effect. However, the medium weight molecules harboring a functional group form additional cross-linking structures, so the concentration of stress due to the contraction of a polarizing plate is still a problem.
Japanese Laid-Open Patent Publication No. 2003-49141 describes a method to reduce light leakage by applying an adhesive composition composed of an acrylic copolymer containing a functional group which has a weight average molecular weight of 1,000,000˜2,000,000, a medium molecular weight acrylic copolymer having less than 2 functional groups which has a weight average molecular weight of 30,000˜300,000, a low molecular weight acrylic copolymer excluding a functional group that has a weight average molecular weight of 1,000˜20,000 (degree of dispersion 1.0˜2.5), and a cross-linking agent to a polarizing plate to have a stress relaxation effect. However, the medium molecular weight molecules having a functional group form an additional cross-linking structure as described above, so the stress concentration caused by contraction of a polarizing plate is still a problem to solve. In addition, if the low molecular acrylic copolymer having a molecular weight of less than 20,000 is used for a long time, surface migration will be observed with reduced durability.
Japanese Laid-Open Patent Publication No. 2002-12521 describes an effort to reduce the stress concentration by using an adhesive composition for a polarizing plate which is composed of 100 weight part of a high molecular weight acrylic copolymer having a functional group which has a weight average molecular weight of 1,000,000˜2,500,000, 10˜100 weight part of a low molecular weight acrylic copolymer having a weight average molecular weight of 30,000˜100,000, a multifunctional cross-linking compound containing at least two functional groups, and a silane compound. However, in this description only the weight average molecular weights of the acrylic copolymers have been given, without an explanation of their effect.
In general, the physical behavior of a polymer does not simply depend on the weight average molecular weight, it depends more on the relative size to the entanglement molecular weight. So, the composition of an acrylic copolymer has to be determined by considering the entanglement molecular weight. The modulus and viscosity of a polymer, two major factors affecting the stress relaxation property, are rapidly changed when the molecular weight of the polymer is two or three times larger than the entanglement molecular weight. The entanglement molecular weight of (meth)acrylic acid alkyl ester depends on the carbon number of the alkyl group therein. Therefore, when a low molecular weight molecule is added in order to enhance the stress relaxation capacity, the entanglement molecular weight of the low molecular weight molecule has to be considered. If the molecular weight of a low molecular weight molecule is lower than the entanglement molecular weight, an adhesive will flow out of the section when a polarizing plate is cut, or the polarizing plate will be easily contaminated by the discharged adhesive. Besides, the low molecular weight molecules are apt to move to the interface between the glass board of a liquid crystal panel and TAC, resulting in the decrease of durability under high temperature and high humidity conditions.
Therefore, a novel adhesive for a polarizing plate has been needed that is able to improve the cutting-efficiency without reducing durability under high temperature and high humidity conditions and long-term use, and to reduce light leakage of a polarizing plate using the same.